Copper-niobium alloy and method for the production thereof

ABSTRACT

The invention relates to the field of materials engineering and relates to copper-niobium alloys which can be used for producing powder metallurgical products by known shaping methods, e.g., after having been processed into semi-finished products or shaped bodies, and a method for the production thereof. The object of the invention is to disclose copper-niobium alloys and a method for the production thereof in which a homogenous metastable Cu mixed crystal is present and a method for its implementation. The object is attained through a copper-niobium alloy in which, in addition to a copper-niobium mixed crystal, niobium deposits with particle diameters of 5-100 nm are also present in a copper matrix. The object is further attained through a method for producing copper-niobium alloys in which copper powder as matrix material and 0.1 to 50 at. % niobium powder are jointly ground and mechanically alloyed and then subjected to at least one thermal treatment.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of International Application No. PCT/DE03/00764, filed Mar. 3, 2003, and claims priority under 35 U.S.C. § 119 of German Patent Application No. 102 10 423.9 filed on Mar. 4, 2002. Moreover, the disclosure of the International Patent Application No. PCT/DE03/00764 is expressly incorporated by reference.

FIELD OF ART

The invention relates to the field of materials engineering and relates to copper-niobium alloys which can be used for producing powder metallurgical products by known shaping methods, e.g., after having been processed into semi-finished products or shaped bodies, and a method for the production thereof.

PRIOR ART

In order to produce a metallic material with the highest possible mechanical strength and electrical and thermal conductivity, in addition to silver, which has the highest electrical conductivity of all metals, copper is mainly used as a matrix metal because of its much lower cost. In order to increase the strength of the ductile copper matrix effectively, without its thermal and electrical properties being substantially impaired, a strengthening through largely insoluble secondary phases in thermodynamic equilibrium suggests itself. In the case of cubic face-centered copper matrix these can be hard ceramic particles (e.g., oxides, nitrides, carbides), but also a number of high-melting cubic space-centered refractory metals (e.g., Cr, W, Ta, Nb, Mo). Due to their insolubility in the matrix, the above-mentioned alloy additives thereby all meet the requirement for the best possible microstructural stability at high stress temperatures. However, due to the difference in density and melting point present, a homogenous distribution of the respective secondary phase and the particle size distribution in the nanometer range necessary for an efficient increase in strength cannot be achieved through metallurgical fusion methods. The use of the mechanical alloying method overcomes these problems and renders possible, e.g., the production of Ag-oxide composite materials with the finest oxide distribution (diameter <50 nm) and the greatest hardness and strength (B. J. Joshi et al., Proceedings Vol. 3, Powder Metallurgy World Congress, Granada/Spain, 1998; JP 07173555 A; DE 199 53 780 C1).

With mechanical alloying the grinding progress results from repeated fracturing and cold-welding of the powder particles. However, in the case of hard ceramic particles in a soft metallic matrix of copper, when the oxide content is reduced below 10% by volume, the tendency increases that the oxides are only encased by the soft matrix, which means their further fracturing is prevented (DE 44 18 600 C2). The grinding progress is advantageously influenced by a “soft/soft” material combination, i.e., by the selection of a secondary phase best matched to the elastic properties of the copper (C. C. Koch, Nanostructured Materials 2, 1993, 109-129). Of the suitable alloy elements listed above, niobium is the most suitable (L. G. Fritzemeier, Nanostructured Materials 1, 1992, 257-262). The mechanical alloying of concentrated Cu—Nb alloys at room temperature has already been reported (A. Benthalem et al., Scripta Metallurgica et Materialia 27, 1992, 739-744 and Materials Science and Engineering A161, 1993, 255-266). Through the grinding at high energy, the mixed crystal range is expanded and a high homogeneity of the microstructure is achieved. It was likewise shown that part of the added niobium is forcibly dissolved in the copper lattice by the grinding.

However, due to the increase in temperature during grinding, with such alloy systems in general there is a danger of the powders adhering to the wall of the vessel and to the balls, which leads to a low powder yield. Although organic auxiliary agents can reduce the weld tendency, they are fractured by the energy input into CO₂ and H₂ and ground into the powders (U.S. Pat. No. 5,322,666). If this gas is not removed by degassing annealing at high temperatures, a consolidation of the powders into dense compact semi-finished products is not possible or leads to shape instability (“swelling”) during later use at high temperatures. On the other hand, the nanocrystalline grain structure intentionally established in the powder is destroyed through an upstream degassing annealing.

DISCLOSURE OF THE INVENTION

The object of the invention is to disclose copper-niobium alloys and a method for the production thereof in which a homogenous metastable Cu mixed crystal is present and a method for the implementation thereof.

The object is attained through the invention disclosed in the claims. Further developments are the subject of the subordinate claims.

With the copper-niobium alloy according to the invention, in addition to a copper-niobium mixed crystal, niobium deposits with particle diameters of 5-100 nm are present in a copper matrix.

The niobium is thereby advantageously present partially dissolved in the copper lattice.

The niobium deposits are likewise advantageously present in the form of fine particles or fibers.

It is also advantageous if the fibers are present with an aspect ratio of greater than 4:1, advantageously greater than 10:1.

Furthermore, the copper-niobium alloy according to the invention advantageously features a conductivity of 50 to 80% IACS and/or strengths of 1200 to 2000 MPa.

With the method according to the invention for producing copper-niobium alloys, copper powder as matrix material and 0.1 to 50 at. % niobium powder are jointly ground and mechanically alloyed and then subjected to at least one thermal treatment.

Advantageously 0.5 to 20 at. % niobium powder is added.

The grinding process is also advantageously carried out at temperatures of −196° C. to −10° C.

It is also advantageous if the cooling of the grinding vessel is carried out during grinding and/or between the grinding stages.

Furthermore it is advantageous if the grinding vessel is cooled with liquid nitrogen or with ethanol.

It is furthermore advantageous if a complete forcible solution of the niobium in the copper lattice is carried out during grinding.

It is likewise advantageous if the thermal treatment is carried out at temperatures >500° C.

Advantageously the thermal treatment is also carried out at the same time as a shaping process.

Likewise advantageously through the shaping process a fiber-shaped structure of the copper-niobium alloy is produced in which again advantageously a fiber aspect ratio of greater than 4:1, advantageously greater than 10:1 is established during shaping.

It is also advantageous that a high powder yield of the copper-niobium alloy is produced.

It is also advantageous if grinding is carried out for between 20 and 30 hours.

In the method according to the invention at the start an intentional embrittlement of the copper powder is created. In contrast to the materials produced in a known manner by means of mechanical alloying, it is thus possible to obtain a homogenous single-phase alloy of copper with niobium. Since the grinding process is advantageously carried out at low temperatures, it is possible to achieve a partial or complete forcible solution of the niobium atoms in the copper mixed crystal with a relatively low energy input. The degree of solution of the niobium atoms in the copper mixed crystal depends, i.a., on the duration of grinding and the oxygen content of the powders used.

The gas content virtually does not change at all thereby during the grinding process so that an additional degassing step can be omitted during further processing.

After the niobium atoms have been dissolved in the copper mixed crystal, fine niobium particles are deposited during the subsequent thermal treatment. These niobium deposits substantially contribute to increasing the strength and ensuring a high conductivity of the alloy.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention is described below in more detail on the basis of an exemplary embodiment.

A grinding cup charged with steel balls, copper powder and 10 at. % niobium powder is cooled in liquid nitrogen to a temperature on the lid of the grinding cup of −196° C. Subsequently the cooled grinding cup is covered with polystyrene to insulate it from the ambient air. The subsequent grinding process is regularly interrupted every 30 min. in order to cool the grinding cup again to −196° C. The grinding process takes place at a rotational speed of 200 rpm and with a powder/ball ratio of 1:14. In 35 h the added niobium powder has been completely forcibly dissolved in the copper lattice. This can be seen from x-ray and transmission electron microscope (TEM) tests in which niobium reflections no longer occur (FIGS. 1 and 2). The mechanically alloyed powder features a nanocrystalline structure with crystallite sizes of 7 nm. The powder microhardness thereby reaches 500 HV 0.025.

The powder obtained is compacted at 500° C. in a hot press under vacuum and a pressure of 650 MPa to a relative density of 98-99% to form cylindrical shaped bodies with a diameter of 10 mm.

Shaped bodies produced in this way have a hardness of 400 HV 2 with an electrical conductivity of 20 MS/m (corresponding to 35% IACS).

A subsequent thermal treatment during which the niobium is deposited from the copper matrix leads to an increase in electrical conductivity up to 45-50 MS/m (corresponding to 70-80% IACS) with a slight reduction in hardness to 380 HV 2. Through the formation of a fiber structure a final cold shaping increases the hardness to 550 HV 2 again with continued high electrical conductivity.

With a reduction of the Nb content in the starting powder mixture to 5 at. %, mechanically alloyed powder can be produced with a crystallite size of 11 nm and a powder microhardness which reaches 450 HV 0.025. At 30 MS/m (50% IACS) the electrical conductivity of the shaped bodies is then somewhat higher and at 350 HV 2 the hardness somewhat lower than with the shaped bodies with 10 at. % Nb. By carrying out a thermal treatment a higher electrical conductivity of approx. 50 MS/m (80% IACS) is achieved, but also a lower hardness of approx. 300 HV 2, which can be increased to 500 HV 2 again through the final shaping.

Reference Numbers:

FIG. 1 a Diffraction image of the Cu/20 at. % Nb powder, 17 h grinding time

FIG. 1 b: Diffraction image of the Cu/10 at. % Nb powder, 35 h grinding time

FIG. 2 X-ray analysis of the mechanically alloyed powders according to the different grinding times. 

1. Copper-niobium alloy comprising in a copper matrix, in addition to a copper-niobium mixed crystal, niobium deposits with particle diameters of 5-100 nm.
 2. Copper-niobium alloy according to claim 1, wherein the niobium is present partially dissolved in the copper lattice.
 3. Copper-niobium alloy according to claim 1, wherein the niobium deposits are present in a form of fine particles or fibers.
 4. Copper-niobium alloy according to claim 3, wherein the niobium deposits are present in a form of fibers, and the fibers have an aspect ratio of greater than 4:1.
 5. Copper-niobium alloy according to claim 1, with a conductivity of 50 to 80% IACS.
 6. Copper-niobium alloy according to claim 1, with a strength of 1200 to 2000 MPa.
 7. Method for producing a copper-niobium alloy according to claim 1, wherein copper powder as matrix material and 0.1 to 50 at. % niobium powder are jointly ground and mechanically alloyed and subsequently subjected to at least one thermal treatment.
 8. Method according to claim 7, wherein 0.5 to 20 at. % niobium powder is added.
 9. Method according to claim 7, wherein the grinding is carried out at temperatures of −196° C. to −10  C.
 10. Method according to claim 7, wherein the grinding is performed in a grinding vessel, and cooling of the grinding vessel is carried out at least one of during grinding and between grinding stages.
 11. Method according to claim 7, wherein the grinding is performed in a grinding vessel, and the grinding vessel is cooled with liquid nitrogen or with ethanol.
 12. Method according to claim 7, wherein a complete forcible solution of the niobium in the copper lattice is carried out during grinding.
 13. Method according to claim 7, wherein the at least one thermal treatment is carried out at temperatures ≧500° C.
 14. Method according to claim 7, wherein the at least one thermal treatment includes a thermal treatment carried out at a same time as a shaping process.
 15. Method according to claim 14, wherein the shaping process includes forming a fiber-shaped structure of the copper-niobium alloy.
 16. Method according to claim 15, wherein a fiber aspect ratio of greater than 4:1, is established during shaping.
 17. Method according to claim 7, wherein grinding is performed for between 20 and 30 hours.
 18. Copper-niobium alloy according to claim 4, wherein the fibers have an aspect ratio of greater than 10:1.
 19. Method according to claim 16, wherein a fiber aspect ratio of greater than 10:1 is established during shaping.
 20. Copper-niobium alloy according to claim 1, comprising a homogeneous single-phase alloy of copper and niobium. 